You will notice when looking at the moon’s surface dark and light areas.
The dark areas are called maria. There are several prominent maria.

Mare Tranquilitatis (Sea of Tranquility): where the first astronauts landed

Mare Imbrium (Sea of Showers): the largest mare (700 miles or 1100 kilometers in diameter)

Mare Serenitatis (Sea of Serenity)

Mare Nubium (Sea of Clouds)

Mare Nectaris (Sea of Nectar)

Oceanus Procellarum (Ocean of Storms)

The remainder of the lunar surface consists of the bright hilly areas called terrae.
These highlands are rough, mountainous, cratered regions.

The Apollo astronauts observed that the highlands are generally about 4 to 5 km (2.5 to 3 miles) above the average lunar surface elevation, while the maria are low-lying plains about 2 to 3 km (1.2 to 1.8 miles) below average elevation.

The Moon makes a complete orbit around the Earth every 27.3 days (the orbital period), and the periodic variations in the geometry of the Earth–Moon–Sun system are responsible for the phases of the moon, which repeat every 29.5 days (the synodic period).

Sides of the Moon

Locked Lunar Phase with earth

The Moon is in synchronous rotation, which means it rotates about its axis in about the same time it takes to orbit the Earth. This results in it keeping nearly the same face turned towards the Earth at all times. The Moon used to rotate at a faster rate, but early in its history, its rotation slowed and became locked in this orientation as a result of frictional effects associated with tidal deformations caused by the Earth.

The side of the Moon that faces Earth is called the near side, and the opposite side the far side. The far side is often inaccurately called the “dark side,” but in fact, it is illuminated exactly as often as the near side: once per lunar day, during the new moon phase we observe on Earth when the near side is dark. The far side of the Moon was first photographed by the Soviet probe Luna 3 in 1959. One distinguishing feature of the far side is its almost complete lack of maria.

Gravity of the Moon and Mass Concentration

Mass concentration or mascon is a region of a planet or moon’s crust that contains a large positive gravitational anomaly.
In general, the word “mascon” can be used as a noun to describe an excess distribution of mass on or beneath the surface of a planet (with respect to some suitable average), such as Hawaii. However, this term is most often used as an adjective to describe a geologic structure that has a positive gravitational anomaly, such as the “mascon basins” on the Moon.

Type examples of mascon basins on the Moon are the Imbrium, Serenitatis, Crisium and Orientale impact basins, all of which possess prominent topographic lows and positive gravitational anomalies. Examples of mascon basins on Mars include the Argyre, Isidis, and Utopia basins. Theoretical considerations imply that a topographic low in isostatic equilibrium would exhibit a slight negative gravitational anomaly. Thus, the positive gravitational anomalies associated with these impact basins indicate that some form of positive density anomaly must exist within the crust or upper mantle that is currently supported by the lithosphere. One possibility is that these anomalies are due to dense mare basaltic lavas, which might reach up to 6 kilometers in thickness for the Moon. However, while these lavas certainly contribute to the observed gravitational anomaly, uplift of the crust-mantle interface is also required to account for its magnitude. Indeed, some mascon basins on the Moon do not appear to be associated with any signs of volcanic activity, suggesting that the mantle uplift might even be super-isostatic (that is, uplifted above its isostatic position). It should be noted that the huge expanse of mare basaltic volcanism associated with Oceanus Procellarum does not possess a positive gravitational anomaly.

The lunar mascons alter the local gravity in certain regions sufficiently that low and uncorrected satellite orbits around the Moon are unstable on a timescale of months or years. This acts to distort successive orbits, causing the satellite to ultimately impact the surface. The lunar mascons were discovered by Paul M Muller and William Sjogren of the NASA Jet Propulsion Laboratory (JPL) in 1968[1] from analysis of the highly precise navigation data from the unmanned pre-Apollo Lunar Orbiter spacecraft. At that time, one of NASA’s highest priority “tiger team” projects was to explain why the Lunar Orbiter spacecraft being used to test the accuracy of Project Apollo navigation were experiencing errors in predicted position of ten times the mission specification (2 kilometers instead of 200 meters). This meant that the predicted landing areas were 100 times as large as those being carefully defined for reasons of safety. Lunar orbital effects resulting from strong gravitational perturbations were ultimately revealed as the cause. William Wollenhaupt and Emil Schiesser of the NASA Manned Spacecraft Center in Houston then worked out the “fix” that was first applied to Apollo 12 and permitted its landing within 300 meters of the target, the previously-landed Surveyor 3 spacecraft.